I think Flexbox specifically is around about the time a lot of people last looked at CSS and it’s actually everything after that that I was referring to.

I think this resource is pretty well put together from the Chrome dev rel folks https://web.dev/learn/css it obviously goes from 0 so you can pick and choose what parts make sense for you but based on both my own experience and after having talked to many others in the same scenario, if you feel like you never really “got” CSS with any sense of confidence I’d say start at maybe reviewing the box model and going on from there precisely because so much has changed all over the place since then.

How we think about layout is totally different, how we think about positioning is different, how we measure things is new, even things like colour have totally changed in big ways.

But more concretely I think a lot of the reasons that people had particularly bad experiences with CSS in the past that drove them towards things like this or Tailwind for example are now solved problems and that both this and Tailwind are kind of obsolete in many senses.

Keep in mind that this system while it was released publicly today is actually rather old technology and was built around solving a problem in a very specific way that makes a lot less sense now because it was solving problems that now have native CSS solutions with great cross browser support.